The present invention relates to a device for drying and sintering metal-containing ink on a substrate, comprising multiple optical radiators for irradiating the substrate and a reflector for reflecting radiation onto the substrate, wherein the radiators and substrate are movable relative to each other in a transport direction.
Devices for drying and sintering metal-containing inks in the sense of the invention are used for curing printed layers; they are used, for example, for manufacturing electronic circuit elements, in particular, for manufacturing RFIDs, organic photovoltaics, OLEDs, or printed batteries.
It is known that electronic circuit elements can be produced easily and economically by printing methods. Such electronic circuit elements are therefore also designated as “printed functionalities” or “printed electronics.”
In the production of printed electronics, the metal-containing ink is first deposited in a first processing step by a printing process as a thin layer on a suitable substrate, for example a plastic film, paper, or glass. The thickness of the ink layer is usually between 0.3 μm and 3.0 μm. For depositing the ink layer, a number of different printing methods can be used. Screen printing, a roll-to-roll process, or an ink jet printing method is often used. With ink jet printing, the metal-containing ink is transferred drop by drop from an ink-jet printer onto the substrate. This belongs to digital printing methods, because this method creates each image anew.
Inks that are used for the production of printed electronics contain a high percentage of small metal particles, whose particle sizes often lie in the nanometer range. The metal particles are dispersed in an aqueous or organic dispersant. In addition, the inks can contain organic additives, for example for better particle crosslinking, solubility, wettability, or for preventing agglomeration, but also aqueous additives for better processability of the inks.
To produce an electrically conductive and permanent coating of the substrate, it is necessary to dry and sinter the ink coating in a second processing step. The drying process that takes place first contributes to removing volatile components of the ink, for example organic additives or the dispersant. In the metal particle coating remaining on the substrate, however, the metal particles are initially present as individual particles, which must be connected to each other by a subsequent sintering process, in order to produce a conductive coating. High electrical conductivity is achieved, for example, with inks containing nanoparticles made of silver, which have a solids content of approximately 50%.
For the production of printed electronics, a number of drying and sintering processes are used. For example, the ink coating can be dried by the use of heated gases. As gases, for example, air, nitrogen, or noble gases are suitable. However, because substrates made of plastic having a limited thermal stability are often used for the production of printed electronics, the temperature of the heated gases cannot be selected arbitrarily high. Therefore, the use of heated gases usually only dries the coating. The use of gases for drying coatings also has large spatial requirements and leads to long process times.
To nevertheless ensure a quick drying and sintering process, radiation sources are used for irradiating the coated substrate with optical radiation. For these devices, both UV radiation sources, for example mercury vapor discharge lamps or LEDs, are also used as radiation sources that emit NIR radiation, for example pulsed xenon or krypton flash lamps or NIR laser diodes.
For example, U.S. Patent Application Publication 2010/0003021 A1 teaches a device for drying and sintering metal-containing ink with a single radiation source, which is suitable for emitting radiation having wavelengths in the visible, infrared, and/or UV range, for example in the form of a xenon flash lamp. In addition, the device comprises a control unit for controlling the radiation as a function of the optical properties of the cured coating.
However, xenon flash lamps have some disadvantages; they generate periodic radiation having a high power density and are relatively expensive. In addition, for the use of a xenon flash lamp, the irradiation field is not constantly irradiated. In particular, for a quick relative movement of the substrate and flash lamp, the irradiation field might be illuminated inhomogeneously. This can have an effect on the uniform evaporation of volatile components of the metal-containing ink.
It has also been shown that the use of a single radiation source for the simultaneous drying and sintering of metal-containing ink can lead to an incompletely developing sintering process. Simultaneous drying and sintering processes can have the result that surface sintering starts before volatile components have completely evaporated from deeper lying layers of the coating. If these volatile components are further heated during the joint drying and sintering process, microscopic explosions can occur, which might damage the already sintered layer. Such explosions negatively affect the conductivity of the coating.
To prevent this disadvantage, it is proposed in U.S. Patent Application Publication 2013/0043221 A1, from which a device of the type mentioned at the outset is known, to provide a first flash lamp of lower irradiation power for evaporating volatile components and to arrange, downstream of this lamp viewed in the transport direction of the substrate, a separate second flash lamp having higher irradiation output power for sintering the coating.
A separate arrangement of two radiation sources requires a large installation space, in which the radiation sources are arranged; it negatively affects a compact construction of the device. A device having several flash lamps is also complicated to produce and contributes to high manufacturing costs. Flash lamps also have the disadvantages mentioned above; they are expensive and normally designed for high radiation densities.